The present invention relates to rear projection screens for slide or movie projectors, and in particular to an improved Fresnel lens and diffuser system for such projectors.
Images on photographic films can be projected on a screen with devices such as slide projectors or movie projectors. Electronic images can also be projected on screens through the use of high power cathode ray tubes. This is the principle used in most projection televisions sold today. More recently, with the advancement of liquid crystal displays (LCD), compact projectors have been developed and have led to a new generation of low cost and high performance projection televisions. As a result, there is an increasing demand for low cost rear projection screens. The common characteristics of such viewing screens are high light efficiency, wide viewing angle and uniform brightness.
There is a tradeoff between wider viewing angles and brightness of the image at any viewing angle. In particular, it is desirable to have a wide viewing angle in the horizontal direction so that people can be seated on either side. However, since most people's eyes are at similar levels vertically, it is desirable to have a narrower viewing angle vertically to preserve the brightness of the image.
FIG. 1 illustrates the principle of an image projection system 100. An image projector 101 projects images on a rear projection screen 102. The light coming out from the projector subtends an angle 103 and angle 104 depicts the diffusing angle of the screen. The angle 104 also defines the angle from which a viewer can see the images on the screen.
FIG. 2 illustrates a basic structure of a rear projection screen 200. Element 202 is a field lens. Its function is to collimate the diverging cone of light 203 into a parallel beam of light. Element 202 can be a glass lens. But, more commonly, it is a plastic Fresnel lens. Element 205 is a diffuser which causes the incident light falling on its surface to spread over an angular cone. Angle 204 depicts the diffusing angle on the plane of the paper and angle 206 depicts the diffusing angle on a plane out of the paper.
FIG. 3 shows a rear projection screen according to U.S. Pat. No. 4,773,731. In this prior art, the Fresnel lenses are recorded on both surfaces of a first element 302. Element 302 is bonded to a second element 320 which contains scattering particles 318 and surface structure 322 to increase the viewing angle. The refractive index of 302 is different from the refractive index of 320.
FIG. 4 shows another construction of the field lens according to U.S. Pat. No. 6,046,847. The alternating zones of the Fresnel lens have different focal lengths f1 and f2. As a result, the light beam from the projector can be made parallel in one direction and converging in a direction orthogonal to the first direction.
FIG. 5 shows another embodiment of rear projection screen where special structures and Fresnel lens are molded into a single plastic element (U.S. Pat. No. 6,304,378).
FIG. 6 shows a diffusing element consisting of micro-lenses and light blocking elements according to U.S. Pat. No. 5,870,224. The light blocking elements enhance the contrast in the projected image. A Fresnel lens is made by first mechanically cutting circular grooves with different slopes on a metal blank. After machining, this blank is used in a plastic injection molding equipment to replicate the structure onto a plastic sheet. The depth of the grooves in a mechanically engraved Fresnel lens is typically more than 100 μm and is not suitable for use in surface embossing equipment.